Add G to comparison arrays. Not a great test, as this is zero for MTait, but at least checks it is set...
Add arithmetic and harmonic averages for G in solidsolution (thanks to Mark Panning for pointing out they were missing)
Use harmonic mean for now
Check for zero shear modulus
Merge pull request #93 from ian-r-rose/add_shear_to_solidsolution

Add shear to solidsolution